The bZIP transcription factor Nrf2 controls a genetic program that protects cells from oxidative damage and maintains cellular redox homeostasis. Keap1, a BTB-Kelch protein, is the major upstream regulator of Nrf2 and controls both the subcellular localization and steady-state levels of Nrf2. In this report, we demonstrate that Keap1 functions as a substrate adaptor protein for a Cul3-dependent E3 ubiquitin ligase complex. Keap1 assembles into a functional E3 ubiquitin ligase complex with Cul3 and Rbx1 that targets multiple lysine residues located in the N-terminal Neh2 domain of Nrf2 for ubiquitin conjugation both in vivo and in vitro. Keap1-dependent ubiquitination of Nrf2 is inhibited following exposure of cells to quinone-induced oxidative stress and sulforaphane, a cancer-preventive isothiocyanate. A mutant Keap1 protein containing a single cysteine-to-serine substitution at residue 151 within the BTB domain of Keap1 is markedly resistant to inhibition by either quinone-induced oxidative stress or sulforaphane. Inhibition of Keap1-dependent ubiquitination of Nrf2 correlates with decreased association of Keap1 with Cul3. Neither quinone-induced oxidative stress nor sulforaphane disrupts association between Keap1 and Nrf2. Our results suggest that the ability of Keap1 to assemble into a functional E3 ubiquitin ligase complex is the critical determinant that controls steady-state levels of Nrf2 in response to cancer-preventive compounds and oxidative stress.

No abstract

A kinase distinct from the MEK activator Raf, termed MEK kinase-1 (MEKK), was originally identified by virtue of its homology to kinases involved in yeast mating signal cascades. Like Raf, MEKK is capable of activating MEK in vitro. High-level expression of MEKK in COS-7 cells or using vaccinia virus vectors also activates MEK and MAPK, indicating that MEKK and Raf provide alternative means of activating the MAPK signalling pathway. We have derived NIH3T3 cell sublines that can be induced to express active MEKK. Here we show that induction of MEKK does not result in the activation of MAPK, but instead stimulates the stress-activated protein kinases (SAPKs) which are identical to a Jun amino-terminal kinase. We find that MEKK regulates a new signalling cascade by phosphorylating an SAPK activator, SEK1 which in turn phosphorylates and activates SAPK.

Interaction of the p55 tumor necrosis factor receptor 1 (TNF-R1)-associated signal transducer TRADD with FADD signals apoptosis, whereas the TNF receptor-associated factor 2 protein (TRAF2) is required for activation of the nuclear transcription factor nuclear factor kappa B. TNF-induced activation of the stress-activated protein kinase (SAPK) was shown to occur through a noncytotoxic TRAF2-dependent pathway. TRAF2 was both sufficient and necessary for activation of SAPK by TNF-R1; conversely, expression of a dominant-negative FADD mutant, which blocks apoptosis, did not interfere with SAPK activation. Therefore, SAPK activation occurs through a pathway that is not required for TNF-R1-induced apoptosis.